docs: def _update_keycache(self, *args, forward):
          """Add or remove a key in the set describing the keys that exist."""
          entity, key, branch, turn, tick, value = args[-6:]
          parent = args[:-6]
          kc = self._get_keycache(parent + (entity,), branch, turn, tick, forward=forward)
          if value is None:
              kc = kc.difference((key,))
          else:
              kc = kc.union((key,))
          self.keycache[parent+(entity, branch)][turn][tick] = kc